Younger sibling syndrome strikes again!
It's obvious, then. We'll just say it together. Metroid Prime 2 is not Metroid Prime 1. It's prettier, faster, harder, trickier, and does almost everything better than the first game, but it never actually becomes a sum of its parts as the first did. A lot of Nintendo games feel like one cohesive game, one experience, but for some reason Echoes never reaches that point. That doesn't stop it from being a good game, it's just, well, not as superb as the first.
The main "issue" is that it's harder. People who played Metroid Prime 1 are not guaranteed a free ride in Echoes; while the gameplay remains largely the same, the enemies are much harder and more interesting. Even common enemies have wider ranges of attacks and can take a substantial amount of hits, and the boss battles in particular feel like they were tuned for the hardcore Metroid Prime fan. Unfortunately, this means the game is not built for anyone who thought Samus was pretty cool in Super Smash Brothers; enemies gets more difficult as the game goes on and feels more of an extension of the first than its own standalone experience.
In this case, the game plays more like Majora's Mask in the Zelda series. Nintendo knew that Ocarina of Time would be a huge smash hit and no sequel could compare, so they made an alternate, softcore-need-not-apply experience that wasn't remotely close to the first. But while Majora's Mask was ridiculously pretentious and tried too many things at once, Metroid Prime keeps it to the tried-and-true, light-and-dark-world system that extends back to A Link To The Past, while keeping the gameplay to the same basic idea that was so successful in the original.
The atmosphere is back from Metroid Prime 1, but this time fear is more of an element, and plays a large part in Echoes. While there were scary moments in the first game, they were limited to a few "Metroid attack!" scenes where the lights go out, but they were rarely used other than adrenaline boosts to break the monotony. In the second game, the shadow creatures from the dark realm are something to be feared; this is established from the beginning when Samus gets her butt kicked by Dark Samus. It's a theme that continues onward through the game; many parts, especially in the Dark world, appear so alien and bizarre that they're almost frightening.
This is a place where Echoes one-up's its predecessor. The art design of this game is still excellent; Retro Studios is the best in the business at creating a world that appears so much more vivid than your average video game, and that architecture talent continues throughout Echoes. Monsters have much better animation and fluidity, and the environments still have that sort of eye-opening ability that few games possess. With the new visual powers the game's engine has, the world seems that much more alive, with more realistic textures and landscaping.
The game's flaws, however, mar this potentially wonderful game. Aside from the difficulty, the worlds that inhabit the planet of Echoes never seem very connected, and the overall environment feels slightly too small. The game's plot, as well, has advanced to the point of actual characters, but remains minimal and forces Echoes into a very long and dull key-fetch quest. The split-screen multiplayer, as well, is a very poor addition and is much too limited to offer much over a single battle. These problems may not sound like issues, but in execution the game doesn't even feel quite right in the hands of the player as some other Nintendo games do.
But, at the end of the day, Metroid Prime 2 Echoes is largely an addition of the first game. It starts off harder and stays that way, but continues to advance the series and gameplay in ways that are both natural and encouraging. While it's not as approachable as the first, fans of the first are very much advised to check out Echoes; it may not be as good as the first game, but it's an excellent installment in the Metroid series nonetheless.